Rhys won a momentous victory in 1081 at the battle of Mynydd Carn where he allied with the Gwynedd lord Gruffudd ap Cynan who had recruited a mercenary force from Ireland. The laconic notice in the earliest (circa 1100) text of the Annales Cambriae states that Rhys and Gruffudd defeated Caradog together with his allies Trahaearn ap Caradog (lord of Arwystli, Ardudwy, and Meirionydd and the most powerful prince in northern Wales) and Meilyr ap Rhiwallon of Powys. A more elaborate account of uncertain objectivity comes from the History of Gruffudd ap Cynan where Gruffudd returns from exile in Ireland with a fleet provided by Diarmait son of Enna, a grandson of the great Diarmait mac Máel na mbó. He lands at Porthclais, near the Cathedral of St. Davids, and is met by Rhys, who claims that he was expelled from the kingship of Deheubarth and found sanctuary in the cathedral community. After acknowledging Gruffudd as his overlord and promising half his kingdom, the two men lead their supporters to victory in battle against Caradog ap Gruffudd, the men of Gwent, Glamorgan, the Normans, Meilyr ap Rhiwallon and Trahaearn of Arwystli. The alliance did not survive the triumph and Rhys departed secretly fearing treachery from Gruffudd who showed his displeasure by ravaging Deheubarth. Rhys's victory upset the Norman tactic of ‘divide and control’ in South Wales and probably was at least partly responsible for the famous ‘pilgrimage’ to St. David's by King William I of England in that year. Modern opinion believes that Rhys submitted to William, but no medieval record is aware of it, although the statement in the Herefordshire Domesday that Rhys rendered £40 in tax to William argues for some acquiescence.
Challenges to Rhys's rule revived following William's death. He was temporarily expelled from his kingdom in 1088 by three nobles from Powys: Madog, Cadwgan, and Rhirid, the sons of Bleddyn ap Cynfyn. Rhys fled to Ireland and recruited a mercenary force. Upon his return to Wales later that year, Rhys met his enemies at a place called variously Portlethern or Llech y crau, where he was victorious, killing Madog and Rhirid. Three years later Rhys defeated and killed a distant cousin named Gruffudd ap Maredudd (who had been living on his estates in Herefordshire) in a battle at Llandudoch (St. Dogmaels). His end came during Easter week 1093 when Rhys was attacked and killed by the Norman settlers in Brycheiniog led by Bernard de Neufmarché, who was the husband of Gruffudd ap Llywelyn's granddaughter Nest. Welsh and English chronicles claim that the death of Rhys opened Wales to the advancing Normans.
Rhys is also remembered because of his family connections. According to Achau Brenhinoedd a Thywysogion Cymru, Rhys's wife Gwladus was the daughter of Rhiwallon ap Cynfyn, thereby the cousin of his opponents in 1088. He had three known children: Gruffudd (died 1137) who succeeded his father in southern Wales after a lapse of two decades; Hywel; and a daughter named Nest. His descendants included the historian Gerald de Barri, better known as Gerald of Wales, and Rhys ap Gruffudd (‘the Lord Rhys’) who dominated Wales at the end of the twelfth century. Notwithstanding his slaughter of dynastic competitors, Rhys was remembered as a pious individual and the hermit Caradog of Rhos emerged from his court to begin a life of religious solitude.
Published date: 2016